Airflow Excitation Research Articles

A novel dynamic model on nonlinear vibrations of functionally graded graphene platelet reinforced rotating pretwisted composite blade considering subsonic airflow excitation and blade-casing rubbing

A novel dynamic model of the functionally graded graphene platelet (FGGP) reinforced rotating pretwisted composite blade under the aerodynamic force and blade-casing rubbing is established for the first time. The blade is simplified to a FGGP reinforced rotating pretwisted composite cantilever plate. The dynamic model of the axial excitation produced by considering the aerodynamic force in the tip clearance includes two trigonometric functions with different frequencies. The subsonic airflow is considered as a transverse excitation which is derived by utilizing the vortex lattice method (VLM). The dynamic model expresses the changes of the non-uniform axial and contact forces through the tip clearance and rotating pretwisted plate-casing rubbing when the shaft is eccentric. Based on Rayleigh-Ritz method, the linear frequencies and mode shapes are obtained for the FGGP reinforced rotating pretwisted plate. The influences of the graphene distribution pattern, rotating pretwisted plate-casing rubbing, axial excitation in the tip clearance and geometric parameter on the frequencies are investigated. Using the obtained mode shapes, von-Karman type nonlinear geometric assumptions and Lagrange equation, the differential governing equations of motion are derived for the FGGP reinforced rotating pretwisted plate. The nonlinear vibrations under the 1:1 internal resonance at two critical rotating speeds is studied by using Runge-Kutta method. The amplitude-frequency and force-frequency response curves under the low and high critical rotating speeds are investigated by using numerical calculations. The obtained results demonstrate the influence of the rotating speed, frequency ratios and incoming flow speeds on the nonlinear vibrations of the FGGP reinforced rotating pretwisted plate. The dynamic model of the rubbing-impact for the rotating blade provides a theoretical basis for the blade-casing rubbing analysis. At the same time, this study is also used as a theoretical guidance to reduce the damage of the blade-casing rubbing and blade design.

Theoretical and Experimental Study on Rapid Decompression Oscillation in Altitude Chamber

Given the frequent airflow excitation phenomenon caused by the rapid decompression of the altitude chamber, the mathematical model of the nonlinear system of the rapid decompression of the altitude chamber is established. The polynomial parameter method is used to evaluate the characteristics of airflow oscillation, and a rapid decompression test system is built. The test results verify the pressure oscillation phenomenon of the numerical simulation. This paper proves the phenomenon of fluid-induced vibration in the process of rapid decompression and determines that the main factors affecting the induced oscillation are the diameter of the pipe (throat), pressure difference between the two chambers, and initial pressure conditions. Specifically, this study establishes safety and reliability for preventing engineering accidents caused by resonance in the altitude chamber.

Research on the dynamic characteristics of crack damage of a seal-rotor system

In rotor systems, the labyrinth seal system is the core component to suppress fluid leakage between the rotor and the stator. In this paper, based on the finite element method, a dynamic model of a seal-crack rotor system is established by using the Muzynska nonlinear seal force model and the cosine crack stiffness model, and the vibration characteristics of airflow excitation and single-crack and double-crack coupled faults are analyzed. This paper analyzes the vibration characteristics of the coupling of air-induced vibration and a crack fault. First, numerical simulation analysis and test verification were performed on the system response with no sealing force or crack failure. Subsequently, systems with a sealing force and different crack parameters were analyzed for numerical simulation analysis, and then, the influence of crack damage failure on other sealing parameters (including the sealing pressure difference, sealing gap, and sealing length) was studied. Finally, the influence of double-crack damage (damage location, damage degree, phase difference angle) on the rotor system was analyzed. The results show that when the crack depth increases to a certain value, it causes a superharmonic resonance phenomenon in the subcritical speed region of the system. When the system has a sealing force, the airflow excitation frequency of the system can be affected as the degree of crack damage increases. The coupled dynamic response of airflow excitation and crack faults shows a rich spectrum of nonlinear phenomena, which is closely related to the degree of cracks and sealing parameters. Increasing the crack angle weakens the impact of crack damage on the system. This research provides a theoretical basis for detecting and diagnosing crack faults in labyrinth seal-rotor systems.

Numerical Study on Vibration Response and Fatigue Damage of Axial Compressor Blade Considering Aerodynamic Excitation

Axial compressor blades with a deformed initial torsion angle caused by aerodynamic excitation resonated at the working speed and changed the rule of fatigue damage accumulation. The fatigue life of a blade has a prediction error, even causing serious flight accidents if the effect of torque causing damage deterioration of the blade fatigue life is neglected. Therefore, in this paper, a uniaxial non-linear fatigue damage model was modified using the equivalent stress with torsional shear stress, and the proposed fatigue model including the torsional moment was used to study the compressor blade fatigue life. Then, the blade numerical simulation model was established to calculate the vibration characteristics under complex loads of airflow excitation and a rotating centrifugal force. Finally, the blade fatigue life under actual working conditions was predicted using the modified fatigue model. The results show that the interaction between centrifugal and aerodynamic loads affects the natural frequency, as the frequencies in modes dominated by bending deformation decreased whereas those dominated by torsional deformation increased. Furthermore, the blade root of the suction surface showed stress concentration, but there is an obvious difference of stress distribution and amplitude between the normal stress and the equivalent stress including torsional shear stress. The additional consideration of the torsional shear stress decreased the predicted fatigue life by 4.5%. The damage accumulation rate changes with the loading cycle, and it accelerates fast for the last 25% of the cycle, when the blade fracture may occur at any time. Thus, the aerodynamic excitation increased the safety factor of blade fatigue life prediction.

Sound Radiation Mitigation of Geometrically Nonlinear Plates Subjected to Subsonic Airflow

This paper gives a theoretical analysis to obtain and reduce the acoustic pressure generated from plates with geometric nonlinearities subjected to subsonic airflow and external excitation. von-Kármán assumptions are applied considering the nonlinear terms in strain-displacement relations. Airflow passing through the plate is considered as an incompressible, irrotational, and inviscid flow. Galerkin’s method is employed to acquire the governing equations of time-dependent coefficients. Multiple Time Scale Method (MTSM) is then used to obtain the response of the plate. Suppressing undesirable vibration is carried out using an optimal tuned mass damper (TMD) system and an analytical solution is proposed based on Laplace transform and Adomian Decomposition Method (ADM). The acoustic pressure received from the plate is calculated by solving the Rayleigh integral using Boundary Element Method (BEM). A parametric study is carried out, and the effects of the flow speed, the aspect ratio, the thickness of the plate, the material of the plate, the forcing frequency, and the effectiveness of the designed TMD system on the sound pressure are examined. According to the results, using the TMD system reduces the amplitudes of the plate vibrations and, consequently, reduces the acoustic pressure around the vibrating plate. In this study, the passive control strategy leads to a significant decrease in the sound pressure level (about 35%) in some airflow speeds. Results show good efficiency in the control strategy. It is also found that the acoustic pressure generated from steel plates is significantly larger than that generated from aluminum plates. Moreover, increasing the aspect ratio and the plate thickness reduces the acoustic pressure. On the other hand, the external excitations with lower frequencies and the airflows with higher densities can generate lower sound pressures around the plate.

Dynamic Characteristics Analysis of a Seal-Rotor System With Rub-Impact Fault

AbstractLabyrinth seals are widely used to prevent fluid leakage in high-low pressure areas of the rotating machinery. However, the rub-impact fault easily occurs in labyrinth seals. Considering the influence of gyroscopic effect, a finite element model of seal-rubbing rotor system is established in this study based on the Muszynska seal force model, the rolling bearing force model, and the nonlinear rubbing force model. The vibration characteristics under the coupling faults of airflow excitation and rub-impact are analyzed. First, the response of the system without rub-impact fault is simulated numerically and verified by experiments. Subsequently, the dynamic characteristics of the rotor under the conditions of slight and severe rub-impact faults are analyzed. Finally, the influence of the rub-impact parameters is further studied. The results indicate that when the rub-impact fault is absent, airflow excitation occurs at a certain speed, which exhibits the characteristics of frequency locking and combination frequency. The coupling dynamic responses of airflow-induced vibration and rub-impact fault show a rich spectrum of nonlinear phenomena, which are closely related to the degree of rub impact. This study may provide a theoretical basis for the detection and diagnosis of fluid-induced rub-impact fault in labyrinth seal-rotor systems.

An Experimental Platform with Rotating Airflow Excitation and Stationary Blisk Vibration Measurement

With the question that the vibration of the realistic blisk in the engine is hard to be predicted, researchers need an experimental test platform to study the realistic dynamic properties and monitor the vibration of the designed blisk. For keeping the relative motion between airflow and blisk in a real engine, and facilitating the measurement of the blisk vibration, a new experimental platform with rotating air excitation and stationary blisk vibration measurement is developed in this paper. Firstly, the schematic diagram of the test platform is shown and all the components are illustrated clearly, including the stationary airflow excitation system, the test rig, and the vibration measurement system. Then two typical experiments are conducted with the designed academic blisk. A stationary excitation and a transient acceleration excitation are employed to the stationary blisk. Through the time-frequency analysis of the vibration signal measured by the Laser Doppler Vibrometer, the source of each component of the vibration signal is obtained. The experiment results show that the stationary and transient vibration of the blisk can be measured and identified correctly.

Suppression mechanism of a novel chamber structure on the unsteady aerodynamic force

Partial admission can ensure the efficient operation of the turbine unit while adjusting the aerodynamic load. It has a wide range of application prospects due to its advantages. However, the rotor blades of partial admission turbine are subjected to periodic airflow excitation, which affects the blade fatigue life to a large extent. Based on the control stage model, the optimized chamber structure was proposed to reduce the local aerodynamic exciting force in the previous research. However, the local aerodynamic exciting force was not significantly reduced in the nozzle group linking-up section. Therefore, a novel chamber structure by adding flow cavity at nozzle group linking-up section is proposed to effectively suppress the local aerodynamic exciting force. The three-dimensional unsteady numerical method is employed to analyze the flow parameters of the cascade and the variation of unsteady aerodynamic exciting force. The feasibility and superiority of the proposed novel chamber structure are explained from the flow mechanism. The simulation results show that the high-speed flow caused by the narrow flow space in the nozzle group linking-up section is greatly reduced. Thus, enabling the relief of the pressure sudden change and the promotion of pressure in the low-pressure zone. The local axial aerodynamic exciting force is reduced by up to 59.4%. Meanwhile, the novel chamber structure reduces the rotor inlet pressure value from the block section to the inlet section of the nozzle group linking-up section. The local axial aerodynamic exciting force is reduced by 6.8%. In addition, it is worth noting that the novel chamber structure does not have an obvious impact on the aerodynamic performance. The relative change of efficiency is only 0.91%. From the verification results, it is evident that the novel chamber structure can effectively reduce the local aerodynamic exciting force in the nozzle group linking-up section.

Corrigendum to “Measurement of nonlinear vibration response in aerospace composite blades using pulsed airflow excitation” [Measurement 130 (2018) 422–434

Corrigendum to “Measurement of nonlinear vibration response in aerospace composite blades using pulsed airflow excitation” [Measurement 130 (2018) 422–434

Saturation phenomena and nonlinear resonances of rotating pretwisted laminated composite blade under subsonic air flow excitation

The purpose of the present investigation is to reveal the saturation phenomena and the primary resonance of a rotating pretwisted laminated composite blade subjected to a subsonic airflow excitation in the case of 1:2 internal resonance. The flexible compressor blade is treated as a rotating laminated composite cantilever rectangular plate clamped on the rigid disk with the pretwisted and the preset angles. The subsonic air flow is regarded as the transverse excitation around the finite length of the plate. The subsonic air force is derived by using Vortex Lattice method. The third-order shear deformation plate theory, von Karman geometry nonlinearity and Hamilton principle are utilized to derive the nonlinear partial differential governing equations of motion for the rotating plate subjected to the subsonic aerodynamic force. Chebyshev-Ritz method is used to obtain the natural frequencies of the composite cantilever plate with varying rotating speed. Using Galerkin method, the partial differential governing equations of motion is discretized into a two-degree-of-freedom nonlinear system. The nonlinear torsional-bending coupled vibrations with 1:2 internal response are investigated by the method of multiple scales. The saturation and the jumping phenomena between the torsional vibration mode and the bending vibration mode are investigated for the rotating cantilever plate. Numerical simulations demonstrate that the rotating plate exhibits the complicated nonlinear dynamic behaviors under the effect of the excitation detuning parameter, damping parameter and stiffness coupling coefficients. The energy transfer phenomenon is observed for the composite cantilever plate under the subsonic air flow force.

Investigation of vibroactivity of the mechanical equipment aiming to reduce emitting noise

At present, the noise problem is already widely discussed and identified as a priority to ensure human health and the prevention of occupational diseases. We understand acoustic field management as a targeted use of actions and tools to change field parameters or characteristics. In general, the most effective effects on the fields are the elimination of their generation conditions. In this work the vibration analysis of the recuperators was performed, the limits of the vibration parameters were determined and the correlation between the vibration activity of the device and the sound pressure level in the near environment was determined. After the design of the fan partitions and the airflow excitation of the fans with respect to vibration parameters, it was found that the above mentioned structural changes allowed to reduce the average total weighted sound pressure level in the vicinity by 8%.

Testing of piezoelectric energy harvesters isolated from base vibrations

The influence of structural vibrations on piezoelectric energy harvesters’ performance is already documented in the literature. Testing of cantilevered type piezoelectric transducers was performed in this work, which confirmed that the voltage output of the piezo-film transducers largely used in studies assuming flow-induced excitation is affected by structural vibrations transported to its base. Aiming to exclude the base vibration effect, two piezoelectric energy harvesting (PEH) test rigs isolated from structural vibrations were constructed and tested at the Laboratory of Fluid Mechanics and Turbomachinery in the Aristotle University of Thessaloniki (LFMT, AUTh). In both cases plastic attachments were mounted on the piezo-films to increase their voltage output range. The first set up created air flow excitation using a low power axial fan (set up A) and the second using a centrifugal fan (set up B). The Reynolds numbers ranged at 5000–110,000. The maximum swirl number was 0.5 in both set ups. Traverse mechanisms were employed to study the effect of the harvesters’ distance from the source of excitation on their power output. Both harvesters demonstrated higher power outputs for higher fan speeds. The power output of the harvesters was found to increase with the increase of the voltage root mean square (rms) value. The maximum power harvested was 3.75 μW and it was observed at the highest fan speed of set up B.

A new Fourier series method and displacement function for in-plane vibration and power flow characteristics analysis of isotropic rectangular plates

The generation of in-plane vibration in plates is an important issue and frequently occurs due to the presence of excitations in the ship’s hull due to turbulent fluid flows, turbulent airflow excitation on aerospace structures, gear system subjected to axial excitation, assemblies housing piezoelectric crystals and sandwiched plates, and so on. The present analysis aims to establish a universal and numerically efficient method for determination of in-plane vibration characteristics of isotropic rectangular plates both for conventional and general boundary conditions. The new in-plane Fourier series and displacement function of the plate have been developed using beam displacement functions in x and y directions, respectively, under in-plane condition. A modified Fourier series assumption for the in-plane beam displacement has been utilised and further developed as plate displacement function. The computational efficiency of the present method is compared in terms of convergence of natural frequency parameter, speed of execution and manual convenience to reduce human errors with the frequently used Fourier series method by various researchers. Rayleigh–Ritz procedure has been applied to determine the in-plane natural frequencies. The mode shapes for few conventional and generally varying boundary conditions have been presented and analysed. The dynamic response has been obtained and analysed in terms of the in-plane mobility and power flow characteristics of the plate under varying boundary conditions. The validity of results obtained by the current method has shown excellent accuracy and faster convergence with the existing results. The present results can provide a benchmark to analyse the dynamic in-plane response of plate systems being used for built-up structures in real engineering applications.

Study on Amplitude and Flatness Characteristics of Elastic Thin Strip under Fluid–Structure Interaction Vibration Excited by Unsteady Airflow

Based on unsteady airflow excitation and elastic thin strip vibration theory, a SI-FLAT flatness meter was taken as the research object, and an amplitude–residual stress simulation analysis model of the cold rolling strip under aerodynamic loads was established by using ANSYS Workbench. First, the influences of fluid–structure interaction on the strip amplitude distribution and the flatness calculation deviation were analyzed. It was found that the analysis with fluid–structure interaction matched the actual measurement of the flatness meter better. Then, the influences of different aerodynamic loads and tensions on the strip midpoint amplitude and the flatness calculation deviation were analyzed. It was found that when alternating aerodynamic loads increased, the strip amplitude increased in the form of a quadratic polynomial. However, when the tensions decreased, the strip amplitude decreased exponentially. The strip dimensions also influenced the amplitude of vibration: The wider and thinner the strip, the larger the amplitude. Finally, the influences of different flatness defects on the strip amplitude distribution and the flatness calculation deviation were analyzed. The deviation was serious on the strip edge, and the fluctuation characteristics of the deviation were opposite to those of the initial flatness defects.

Study on Damping Characteristics of Tilting-Pad Gas Seals Based On Random Decrement Method

The influence of the airflow excitation force on the damping characteristics of the seals is an important index to evaluate the stability of seals. In order to verify the stability of the tilting-pad seals proposed by author, in this paper, an impact test is used to test the damping characteristics of the cylinder installed with tilting-pad seals and fixed-pad seals respectively. The multiple hammering signals are processed by the random reduction method to minimize the environmental and anthropogenic interference. Finally, a comparison has been carried out between these two seals under the same experimental conditions, and the result shows that the absolute damping factors of these two seals both decrease with the increase of rotor speed. However, the absolute damping factor of tilting-pad seals is bigger than that of fixed-pad seals, which indicates that the stability of tilting-pad seals is higher than that of fixed-pad seals.

Dynamic Analysis of Flexible Shaft and Elastic Disk Rotor System Based on the Effect of Alford Force

Taken the flexible shaft and elastic disk rotor system as the research object, vibration differential equations are established by the modal synthesis method. Based on the Runge–Kutta method of direct integration, dynamic equations are solved successfully. Then, the effects of Alford force caused by the airflow excitation are researched on the stability and dynamic response. The results show modal vibration of the disk takes a great effect on the coupled natural frequency of shaft transverse vibration in the high rotational speed region. The effect of shaft‐disk coupling makes rotor frequency to decrease, and different rotational speeds have little effect on rotor frequency. Eccentric distance is sensitive to the amplitude of rotor response, but it has little effect on rotor stability, motion law, and relative relationship of two response frequencies. The proportion of rotor responses caused by aerodynamic excitation force becomes larger with the increase of rotational speed and disk radius. However, the proportion decreases with the second support moving to the right, and it is helpful for the stable operation of the rotor system.

The effects of structural parameters on excitation force of airflow vibration piezoelectric generator

In order to meet the requirements of airflow vibration piezoelectric generator, and solve the critical issue of stable excitation force, the key is to effectively control structure sensitive parameters. The range of structural parameters of airflow excitation device are optimized by simplified orthogonal test. The result shows that corresponding to different resonator lengths, it approximates a linear increasing trend between the amplitude of excitation force and airflow velocity, and the greater length owns the smaller slope, and vice versa. It is relatively complex for the space and ring gaps, too big or too small gap would make the amplitude smaller or even no waveform formed. The length of resonator is the main factor impacting on the frequency of excitation sound pressure, and the frequency decreases with the increase of the length, presenting an inverse relationship, and the space and ring gaps have less effect on the frequency. Therefore, for high airflow velocity, the stable excitation force of high amplitude and frequency can be obtained by short resonator. In addition, reasonable space and ring gaps are also important for ensuring bigger amplitude of excitation sound pressure. The resulting sensitive parameter values of sound excitation device can be used as references for engineering design.

Measurement of nonlinear vibration response in aerospace composite blades using pulsed airflow excitation

With increased use of composite materials and more complex designs, nonlinear behaviour of aero engine components has become a more common occurrence. However, both numerical and experimental tools currently available are mostly limited to linear dynamics range. The development of new or improved tools is therefore needed in order to deal more reliably with the identification and quantification of nonlinearities.In this work, an experimental method for the structural testing of composite components in presence of nonlinear vibrations is presented. The method is based on the use of a chopped air-jet vibration exciter providing non-contact input force at the frequency of the pulsating airflow. The non-contact nature of this type of exciter prevents the impedance mismatch issue existing when more traditional systems such as electromagnetic shakers are used. This way, the excitation force is transmitted more efficiently over the whole dynamic response range.Results of forced and free decay response tests carried out on different types of composite blades are presented and discussed. They demonstrate that the proposed vibration exciter provides the capacity to identify geometrical nonlinearities and their effects in all the cases investigated.

Alternating minimisation for glottal inverse filtering

A new method is proposed for solving the glottal inverse filtering (GIF) problem. The goal of GIF is to separate an acoustical speech signal into two parts: the glottal airflow excitation and the vocal tract filter. To recover such information one has to deal with a blind deconvolution problem. This ill-posed inverse problem is solved under a deterministic setting, considering unknowns on both sides of the underlying operator equation. A stable reconstruction is obtained using a double regularization strategy, alternating between fixing either the glottal source signal or the vocal tract filter. This enables not only splitting the nonlinear and nonconvex problem into two linear and convex problems, but also allows the use of the best parameters and constraints to recover each variable at a time. This new technique, called alternating minimization glottal inverse filtering (AM-GIF), is compared with two other approaches: Markov chain Monte Carlo glottal inverse filtering (MCMC-GIF), and iterative adaptive inverse filtering (IAIF), using synthetic speech signals. The recent MCMC-GIF has good reconstruction quality but high computational cost. The state-of-the-art IAIF method is computationally fast but its accuracy deteriorates, particularly for speech signals of high fundamental frequency (F0). The results show the competitive performance of the new method: With high F0, the reconstruction quality is better than that of IAIF and close to MCMC-GIF while reducing the computational complexity by two orders of magnitude.

예혼합 화염 날림 메커니즘에 음향 가진이 미치는 영향에 대한 실험적 연구

In this study, blowoff was investigated in a ducted combustor with the bluff body when acoustic excitation was forced. To observe the flame structure, OH radical chemiluminescence was used and the image was analyzed by using POD (Proper Orthogonal Decomposition) algorithm. Natural gas mainly composed of methane was used as fuel. Blowoff occurred when the equivalence ratio was reduced. Equivalence ratio causing blowoff was measured by changing air flow rate, excitation frequency and sound pressure. Blowoff equivalence ratio was varied depending on the experimental conditions. Vortex frequency behind the bluff body and resonance effect in combustor are the main factors that affect the blowoff equivalence ratios with the excitation.